1. Field of the Invention
The present invention relates to an acrylic pressure-sensitive adhesive composition for a polarizing film, more particularly to a pressure-sensitive adhesive composition for a polarizing film capable of solving the light leakage problem, a polarizing film prepared therefrom and a liquid crystal display device using the same.
2. Description of the Related Art
In general, a liquid crystal cell containing liquid crystals and a polarizing film are basically needed to manufacture a liquid crystal display device. An adequate adhesive layer or a pressure-sensitive adhesive layer is required to join them. In addition, a phase retardation film, a compensation film for wide view angle, a brightness enhancement film, etc. may be further joined to the polarizing film in order to improve the performance of a liquid crystal display device.
Typically, a liquid crystal display comprises a uniformly aligned liquid crystal layer; a polarizing film comprising a liquid crystal cell, which consists of a transparent glass plate or a plastic plate including a transparent electrode layer and an adhesive layer or a pressure-sensitive adhesive layer; a phase retardation film; and additional functional film layers.
The polarizing film contains regularly aligned iodine compounds or dichromic polarizing materials and has a multi-layer structure to protect these polarizing elements by forming protecting films made of triacetyl cellulose (TAC), etc. on both sides of the film. The polarizing film may further comprise a phase retardation film having an anisotropic molecular alignment, a compensation film for wide view angle such as an optically designed liquid crystal film, etc.
Because these films are made of materials having different molecular structures and compositions, they have different physical properties. Especially, under a specific thermal and/or humidity condition, materials having an anisotropic molecular alignment shrink or expand, which causes the lack of dimensional stability. As a result, if the polarizing film is fixed by a pressure-sensitive adhesive, a shear stress caused the thermal and/or humidity condition remains, so that light leakage occurs at the region where the stress is concentrated.
One way to solve the problem of light leakage is to reduce the shrinkage of the polarizing film at the thermal and/or humidity condition. However, it is very difficult to remove the stress applied to a liquid crystal panel to which a polarizing film consisting of different materials has been attached.
Rubbers, acryls and silicones are commonly used as pressure-sensitive adhesives. Among these, acrylic pressure-sensitive adhesives are used the most widely in manufacturing high performance pressure-sensitive adhesive compositions. Molecular characteristics of adhesive materials, such as molecular weight and distribution, cross-linking density, and composition, mainly influence the durability of pressure-sensitive adhesives, which may be controlled by the high adhesive strength and cohesive strength.
However, when a polarizing film manufactured using such a pressure-sensitive adhesive is used for a long time under a specific thermal and/or humidity condition in a liquid crystal display, stress tends to concentrate because of the shrinkage of the polarizing film. To solve this problem, the adhesive layer needs to have a stress relaxation ability.
For this purpose, Japan Patent Publication No. Hei 1-66283 disclosed a polarizing film employing an acrylic pressure-sensitive adhesive layer mainly comprising (meth)acrylic acid alkyl ester having alkyl groups with 1-12 carbon atoms, in which the pressure-sensitive adhesive layer comprises the polymer constituent having a weight-average molecular weight of at most 100,000 in less than 15 wt % and the polymer constituent having a weight-average molecular weight of at least 1,000,000 in more than 10 wt %. Although the polarizing film employing the adhesive comprising a lot of polymer constituent having a high molecular weight has good adhesion durability, it is difficult to relax the stress caused by the dimensional change of the polarizing film because the adhesive has large cohesive force (elasticity).
Korea Patent Publication No. 1998-079266 attempted to solve the problem of light leakage by offering a stress-relaxation property to the pressure-sensitive adhesive composition, which comprises 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight of at least 1,000,000, 20-200 parts by weight of a low molecular weight acrylic copolymer having a weight-average molecular weight of at most 30,000 and 0.005-5 parts by weight of a multifunctional crosslinking agent. However, because of the too high content of the low molecular weight acrylic copolymer having a molecular weight of at most 30,000 and lack of compatibility, it is difficult to ensure durability as bubble and edge lifting at high temperature and high humidity. In particular, the patent did not mention the method of preventing the durability problem caused by surface transference of the material having a low molecular weight after a considerable lapse of time.
Japan Patent Publication No. 2002-47468 attempted to offer stress-relaxation with a pressure-sensitive adhesive composition for a polarizing film comprising 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight ranging from 800,000 to 2,000,000, 5-50 parts by weight of a low molecular weight acrylic copolymer having a with no functional groups and weight-average molecular weight of at most 50,000 or having a degree of dispersion ranging from 1.0 to 2.5 and having no functional groups, a crosslinking agent and a silane compound. However, it did not mention the problems of non-homogeneous adhesion strength and durability after a prolonged time due to surface transference of the material having a low molecular weight and compatibility with the material having a high molecular weight.
Japan Patent Publication No. 2003-49141 attempted to solve the problem of light leakage by offering stress-relaxation with a pressure-sensitive adhesive composition for a polarizing film comprising a high molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000,000 to 2,000,000 and having functional groups, a medium molecular weight acrylic copolymer having a weight-average molecular weight ranging from 30,000 to 300,000 and having less than two functional groups, a low molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000 to 20,000 (degree of dispersion=1.0-2.5) and having no functional groups and a crosslinking agent. While this patent employs the acrylic copolymer having a medium molecular weight and having functional groups, the acrylic copolymer having a low molecular weight and having no functional groups may cause durability problem after a prolonged time because of surface transference. To solve this problem, the acrylic copolymer having a high molecular weight and the acrylic copolymer having a low molecular weight should be compatible with each other. However, this patent did not mention it.
It is also possible to control the crosslinking density by chemical bonding in order to offer stress-relaxation to the pressure-sensitive adhesive. However, it is insufficient to solve the stress concentration problem of the polarizing film. Moreover, if the crosslinking density is too low, the durability problem may occur.
Accordingly, a novel pressure-sensitive adhesive for a polarizing film capable of solving the light leakage problem while maintaining major characteristics of the polarizing film even at high temperature and humidity conditions and after a prolonged time and a polarizing film using the same are urgently needed.